The aim of this work is to determine the DNA sequence organization of genes encoding the major histocompatibility complex (MHC) and the mechanisms controlling the expression of these genes. It has been demonstrated that there are l0-15 MHC genes in the swine genome and 20--30 in the mouse genome. A series of genomic clones containing MCH-homologous DNA sequences has been isolated. One of these clones (PDI) has been studied in detail, both with respect to its DNA sequence organization and regulation of expression. The organization of MHC DNA sequences in chromatin is also under investigation.